1. Field of the Invention
The present invention relates to a power circuit and a communication device provided with the power circuit and more particularly to the power circuit being suitably used in portable cellular phones or a like employing a TDMA (Time Division Multiple Access) or TDD (Time Division Duplex) communication method and to the communication device provided with the power circuit.
The present application claims priorities of Japanese Patent Application No. 2003-030454 filed on Feb. 7, 2003 and No. 2003-183617 filed on Jun. 26, 2003, which are hereby incorporated by reference.
2. Description of the Related Art
In a conventional portable communication device (for example, a portable cellular phone) employing a TDMA or TDD communication method, a transmission signal having a burst period and non-burst period occurring alternately in a repeated manner, after having been amplified by a transmission power amplifier, is transmitted as a transmission radio wave. During the burst period, since the transmission power amplifier transmits a radio wave, power consumption is large and, during the non-burst period, since a radio wave receiving section of the portable cellular phone receives a radio wave, power consumption is small. Therefore, a load current increases or decreases in a burst manner. Moreover, power for the transmission power amplifier is ordinarily supplied from a battery. The battery discharges in synchronization with a repeating cycle of the burst period and non-burst period.
The battery supplies power also to internal circuits such as a CPU (Central Processing Unit) or a like in the portable cellular phone, however, if a voltage of the battery becomes below a lower limit value of an operating voltage of the CPU or a like even momentarily, the CPU or a like is put in a frozen state, causing the portable cellular phone to be inoperable. Therefore, whether or not a residual capacity of the battery exists is judged by detecting a lowest voltage value occurring in various operating states and, by setting a somewhat high terminating voltage obtained by providing a margin based on prediction of a momentary heavy loaded state. Under such conditions, research and development are being conducted to use the portable cellular phone for a longer period of time by expanding a capacity of a battery or by utilizing a DC-DC (Direct Current-Direct Current) converter circuit. Moreover, the TDMA communication method includes a PDC (Personal Digital Cellular) method being used domestically in Japan, a GSM (Global System for Mobile Communications) and/or GPRS (General Packet Radio Service) methods being used in Europe or a like.
Such the conventional portable cellular phone, as shown in FIG. 15, includes a transmission power amplifier 1, a circuit block 2, and a power circuit 3. The transmission power amplifier 1 is made up of an amplifier (AMP) 11, a capacitor 12, an amplifier (AMP) 13, a capacitor 14, an amplifier (AMP) 15, and a bias circuit 16. Each of the amplifiers 11, 13, and 15 is constructed of a bipolar transistor, a MOS (Metal Oxide Semiconductor) transistor, or a like. The bias circuit 16 generates a bias voltage to normally operate these amplifiers 11, 13, and 15. Moreover, each of the transmission power amplifier 1 and circuit block 2 has a lower limit value of an operating voltage required to be operated normally. In the transmission power amplifier 1, a transmission signal RFIN having a burst period and a non-burst period occurring alternately in a repeated manner, which correspond to the GSM communication method, is input to the amplifier 11. The transmission signal RFIN is amplified by the amplifier 11 and an output signal K is then output from the amplifier 11. The output signal K, after its DC (Direct Current) component has been intercepted by the capacitor 12, is input to the amplifier 13 where it is amplified and is output as an output signal M from the amplifier 13. The output signal M, after its DC component has been intercepted by the capacitor 14, is input to the amplifier 15 where it is amplified and a radio wave signal RFOUT as a transmission radio wave from the amplifier 15 is output.
The circuit block 2 includes various circuits each performing specified operations other than amplifying operations to be performed by the transmission power amplifier 1 using almost constant power to be consumed. The various circuits include, for example, a DC-DC converter 21, loads 22 other than the transmission power amplifier 1, or a like, and a lower limit value of a voltage for the specified operations to be performed by each of the various loads 22 is set to be higher than a lower limit value of the voltage for operations to be performed by the transmission power amplifier 1. The DC-DC converter 21 boosts or lowers an output voltage of the power circuit 3. The loads 22 other than the transmission power amplifier 1 includes, for example, a power source for a microcomputer, a power source for a DSP (Digital Signal Processor), a power source for a SIM (Subscriber Identity Module) card, a power source for a memory, a power source for human interface devices (for example, a voice device, an input/output device, an image pick-up device, or a like) and each of the loads 22 is so configured that an output voltage of the power circuit 3 is directly applied to the loads 22 which can operate at the output voltage of the power circuit 3 and the output voltage of the power circuit 3, after being boosted or lowered by the DC-DC converter 21, is applied to the loads 22 which cannot operate at the output voltage of the power circuit 3 and require conversion of the output voltage.
The power circuit 3 is made up of a battery 31, a power management circuit 32, a battery charging circuit 33, and a power bypass condenser 34. The battery 31 is a lithium ion battery and is made up of a single cell 35, an internal resistor 36, and a protective circuit 37. The power management circuit 32 monitors an output voltage of the battery 31 so as to detect a residual capacity and, when the residual capacity of the battery 31 becomes low and when the output voltage reaches a specified reference level having been set to be more than a lower limit value of an operating voltage of the circuit block 2, produces a control signal to display, for example, an alarm indicating a need for charging to notify a fact that the output voltage of the battery 31 has dropped. The battery charging circuit 33 is connected to an outer power source (not shown) charges the battery 31 under specified charging conditions according to a control signal fed from the power management circuit 32. The power bypass condenser 34 delays increasing or decreasing of an output current of the battery 31 occurring at the start time or end time of the burst period.
FIG. 16 is a time chart explaining operations of the conventional portable cellular phone shown in FIG. 15. FIG. 17 is a diagram showing a tolerance of a discharging voltage of the battery 31 shown in FIG. 15, reference for detection of a residual capacity of the battery 31 having been set to the management circuit shown in FIG. 15, a tolerance of an operating voltage of a transmission power amplifier 1 shown in FIG. 15, and a tolerance of an operating voltage of the circuit block 2 shown in FIG. 15.
Next, operations of the portable cellular phone shown in FIG. 15 are described below by referring to FIGS. 16 and 17. As shown in FIG. 15, since the transmission power amplifier 1 is connected to the power circuit 3, an output voltage of the battery 31 becomes equal to an operating voltage of the transmission power amplifier 1. First, at time tα, when a signal transmitting operation is started and the transmission burst period begins, a current to be consumed by the transmission power amplifier 1 sharply increases from a current value 0 A to a current value IPA. An output current of the battery 31 also increases from a current value IB0 to a current value IBmax in synchronization with starting of the transmission burst period, however, increasing of the current is delayed due to a surge absorbing action caused by discharging of the power bypass condenser 34. This serves to suppress a fluctuation of an output voltage of the battery 31 caused by starting of the transmission burst period. An output voltage of the battery 31, due to an increase of its output current occurring at the start time of the transmission burst period and due to existence of a resistance component by serial connection between the internal resistor 36 and the protective circuit 37, drops by a voltage value ΔVBRx (=VB0) from a voltage value VB0 (being equal to an operating voltage VPA0 of the transmission power amplifier 1).
During a period Tβ, that is, during the transmission burst period, since the transmission power amplifier 1 is continuing transmission operations, a current to be consumed remains constant at a level of the current value IPA. The output current of the battery 31, since the delay caused by the power bypass condenser 34 has disappeared, becomes stable at a level of the current value IBmax being a sum of a current consumed by the transmission power amplifier 1 to a current (almost being constant and being equal to the current value IB0) consumed by the circuit block 2. The output voltage of the battery 31, due to a voltage drop corresponding to an electrostatic capacity component of the battery 31 induced by an output current with the value of IBmax, is lowered by a voltage value ΔVBCx (=ΔVCX). Therefore, an amount of change in the output voltage of the battery 31 at an end of the period Tβ, since an amount of voltage drop of ΔVBCx is added to a voltage value ΔVBRx occurring at the time tα, becomes ΔVBx (=ΔVPAx=ΔVBRx+ΔVBCx) and the output voltage of the battery 31 drops from the voltage value VB0 to a voltage value VB1 (=VPA1).
At the time tβ, when the transmission operation is terminated and the transmission burst period ends, the current consumed by the transmission power amplifier 1 sharply lowers from the current value IPA to almost 0A. The output current of the battery 31 decreases to a level of a current (with a value of IB0) consumed by the circuit block 2 in synchronization with ending of the transmission burst period, however, decreasing of the output current is delayed by the surge absorbing action caused by charging of the power bypass condenser 34. This suppresses a fluctuation of the output voltage of the battery 31 caused by ending of the transmission burst period. The output voltage VB1 of the battery 31, since the voltage drop caused by existence of a resistance component by serial connection between the internal resistor 36 and protective circuit 37 decreases in synchronization with ending of the transmission burst period, is boosted by a voltage value ΔVBRx (=ΔVRx).
During a period Tα, that is, during the receiving non-burst period, since the radio wave receiving section of the portable cellular phone is continuing receiving operations and the transmission power amplifier 1 does not operate, a current consumed by the transmission power amplifier 1 is almost 0A. The output current of the battery 31, since the delay caused by the power bypass condenser 34 has disappeared, is stable at a level of the current value IB0 which is a current to be consumed by the circuit block 2. The output of the battery 31, since its output current has sharply decreased from the current value IBmax to the current value IB0, is boosted, based on a time constant, due to existence of a resistance component by serial connection between the internal resistor 36 and protective circuit 37 and due to an electrostatic capacity component of the battery 31.
Then, these voltages and currents are again put into the state that has occurred at the time tα and, thereafter, same operations are repeated in order of the time tα, period Tβ, time tβ, period Tα, time tα, . . . . Thus, by a current consumed by the transmission power amplifier 1 during the transmission burst period, an amount of change in the output voltage of the battery 31 becomes a voltage value ΔVBx and the output voltage of the battery 31 drops from the voltage value VB0 occurring during the receiving non-burst period to the voltage value VB1 which is a lowest level during the transmission burst period. If this voltage value VB1 becomes below a lower limit value of an operating voltage of the internal circuit such as the CPU in the portable cellular phone even momentarily, since the portable cellular phone becomes inoperable, a residual life of the battery 31 is judged based on this voltage value VB1.
When a telephone speech is made using the portable cellular phone, for example, of the GSM type being typical of the TDMA-type portable cellular phone, a voltage value ΔVBx that can be obtained by simulation using following conditions becomes about 300 mV.
Simulation conditions;
Resistance of the internal resistor 36; 150 mΩ
Transmission burst period; 0.5 msec
Receiving non-burst period; 4.5 msec
Output current of the battery 31;
IBmax; 2.1 A,
IB0; 0.1 A.
ΔVBRx=0.15·(2.1−0.1)=0.3 V
ΔVBCx=(0.0005·2.1)/C>0
where “C” is electrostatic capacity of the battery 31.
∴ΔVBx=ΔVBRx+ΔVBCx>300 mV
That is, when the output of the battery 31 is, for example, 3.5V during the receiving non-burst period, it becomes 3.2V or less during the transmission burst period and, since it reaches the level that an alarm indicating a need for charging is issued according to the reference for detection of a residual capacity of the battery 31 shown in FIG. 17, a notification is provided by the power management circuit 32 informing that the output of the battery 31 has dropped.
As shown in FIG. 17, a tolerance of an operating voltage of the transmission power amplifier 1 is 4.2V to 2.7V, a tolerance of an operating voltage of the circuit block 2 is 4.2V to 3.0V and there is a difference of about 0.3V (ΔVM) in the lower limit values in the operating voltage between the transmission power amplifier 1 and the circuit block 2. A reason for this is that the transmission power amplifier 1, since it is constructed of analog circuits, is operable even at a comparatively low voltage, while the circuit block 2, since it is constructed of a CPU and/or digital circuits, is inoperable at a low voltage.
Moreover, in addition to the portable cellular phone described above, a radio communication device as one of examples of the technology described above is disclosed in Japanese Patent Application Laid-open No. Hei 04-315320, in which a capacitor is charged by a battery to have a voltage of 10V using a voltage boosting device and, during a transmission burst period, a switching unit is closed to allow power to be applied by the capacitor to a power amplifier. At this point, a burst signal is amplified by the power amplifier and is transmitted and, during a non-burst period, a switching unit is opened to allow the capacitor to be charged.
However, such the conventional technologies as described above have following problems. That is, even if an output voltage of the battery 31 is, for example, 3.5V during the receiving non-burst period, it becomes 3.2V or less (terminating voltage) during a transmission burst period and, therefore, it reaches a level that an alarm indicating a need for charging is issued and, as a result, a notification is provided by the power management circuit 32 informing that the output voltage has dropped and, before the output voltage of the battery 31 reaches an actual terminating voltage, the conventional portable cellular phone becomes inoperable. To solve this problem, an idea is proposed that a capacity of the battery 31 is increased. However, if the capacity of the battery 31 is increased, it is made impossible to make the portable cellular phone smaller in size and lightweight, which further makes it difficult to meet market needs for the portable cellular phone which enables long-time speech and is compact and lightweight.
Moreover, in the burst radio communication device disclosed in the Japanese Patent Application Laid-open No. Hei 04-315320, power is applied to the power amplifier by the capacitor charged by the battery using the voltage boosting device. However, in many power amplifiers being presently a mainstream, a battery voltage (in the case of the lithium ion battery, it is about 3.7V on average) of the portable cellular phone is applied. Therefore, if a voltage of about 10V is applied by the capacitor, it exceeds a withstand voltage (about 5V) of a power amplifier of the portable cellular phone, which produces a fear that elements within the power amplifier may be broken. Moreover, even when a voltage fed from the capacitor is stepped down by a regulator, since DC/DC converters are used in two stages, another problem arises that power efficiency is remarkably lowered.
Also, in the disclosed burst radio communication device, the switching unit is closed during the transmission burst period and is opened during the non-burst period, however, such the method in which the capacitor is charged at an idle slot time during the non-transmission period (non-burst period) can be employed only in a device in which transmission time is comparatively short (its duty ratio being about ⅛). That is, as a ratio of transmission time becomes larger, the capacitor has to be charged in a shorter time and, when this ratio exceeds 50%, the switching unit produces an adverse effect. In recent years, functions of the portable cellular phone tend to be expanded, that is, although the conventional function is to perform only voice speech, a recent function includes transmission of data. In the portable cellular phone of the TDMA type, such the expansion of the functions causes transmission slots to increase and the ratio of transmission time to rise and, therefore, the switching unit is not effective in achieving long-time speech in the portable cellular phone.
Furthermore, one of the most important performance capabilities of portable communication devices such as portable cellular phones is to be able to provide a satisfactory size and weight that would not hinder a user from carrying them. In recent years in particular, since a folding body of a portable cellular phone is the main stream, it is requested that portable cellular phones are thin and lightweight. However, in the disclosed burst radio communication device, the voltage boosting device is a DC-DC converter made up of a coil, resistor, semiconductor, or a like, the switching unit is made up of a mechanical switching element or a semiconductor switching element, and the capacitor is about 100 mm3 in size. If all these components are housed in the portable cellular phone, the portable cellular phone becomes very large and heavy, thus impairing a portability characteristic of the portable cellular phone.